Referring to FIG. 1, a conventional balance mass system 9, such as those disclosed in U.S. Pat. No. 6,906,786 B2 and U.S. Pat. No. 7,502,103 B2, generally includes a base frame 91, a balance mass 92 elastically coupled to and supported on the base frame 91, a substrate table 93 movably disposed on the balance mass 92 and actuated by an actuator 94 that is connected between the balance mass 92 and the substrate table 93, and a spring damper assembly 95 connected between the base frame 91 and the balance mass 92. During the movement of the substrate table 93 actuated by the actuator 94, a reaction force generated and applied to the balance mass 92 can be counteracted due to the mass of the balance mass 92 being significantly greater than that of the substrate table 93 and that of the actuator 94, and hence an accelerated displacement of the balance mass 92 due to the reaction force is greatly reduced. The displacement of the balance mass 92 can be further reduced by means of the spring damper assembly 95. The balance mass 92 as such is required to have a considerable weight to bear and counteract all reaction force generated by the movement of the substrate table 93 for ensuring stability, which renders the balance mass system 9 bulky and arrangement thereof troublesome.